The Manchester code is a binary encoding which also encodes the clock signal. This is very useful, because an external clock signal is no longer needed. A sequence of bits modulates the phase of the signal. This is a form of Phase shift keying, often shortened to PSK.
Otherwise put, the information about the signal is in its signal flanks, that is when the signal changes.
Manchester codes are widely used, for example for Ethernet or for RFID.